The invention generally relates to metal air electrochemical cells.
Batteries are commonly used electrical energy sources. A battery includes a housing containing a negative electrode, typically called the anode, and a positive electrode, typically called the cathode. The anode contains an active material that can be oxidized; the cathode contains or consumes an active material that can be reduced. The anode active material is capable of reducing the cathode active material. In order to prevent direct reaction of the anode material and the cathode material, the anode and the cathode are electrically isolated from each other by a sheet-like layer, typically called the separator.
When a battery is used as an electrical energy source in a device, such as a hearing aid or a cellular telephone, electrical contact is made to the anode and the cathode, allowing electrons to flow through the device and permitting the respective oxidation and reduction reactions to occur to provide electrical power. An electrolyte in contact with the anode and the cathode contains ions that flow through the separator between the electrodes to maintain charge balance throughout the battery during discharge.
In a metal air electrochemical cell, oxygen is reduced at the cathode, and a metal, such as zinc, is oxidized at the anode. Oxygen is supplied to the cathode from the atmospheric air external to the cell through air access ports in the battery housing. Metal oxide, such as Zinc oxide or zincate, is formed in the anode. Thus, the overall electrochemical reaction within a zinc-air electrochemical cell results in zinc metal being oxidized to zinc ions and oxygen from the air being reduced to hydroxyl ions. While these chemical reactions are taking place, electrons are transferred from the anode to the cathode thereby providing power to the device. An undesirable process can also occur, where the zinc reacts directly with the electrolyte to produce zinc oxide and hydrogen. This not only depletes zinc and electrolyte, but the hydrogen can accumulate to increase internal pressure, damage the cathode and induce leakage. Mercury and other metals such as lead and cadmium often are added to the anode to reduce the levels of hydrogen gas produced during the electrochemical reaction of the battery.
In general, the invention relates to a hydrogen recombination catalyst for metal air electrochemical cells. Metal air electrochemical cells containing hydrogen recombination catalysts have reduced hydrogen gassing. At the same time, the hydrogen recombination catalyst can partially replenish the water content of the electrolyte within the metal-air electrochemical cell, and thereby reduce the amount of drying of the electrochemical cell.
In one aspect, the invention features a metal-air battery including (a) an anode; (b) a cathode including a metal that reduces oxygen; (c) a housing for the anode and cathode having an air access that allows oxygen to contact the cathode; (d) a separator between the anode and the cathode; and (e) a hydrogen recombination catalyst within the housing. The hydrogen recombination catalyst can include Pd, Pt, Ru metals or salts thereof, and CuO.
In another aspect, the invention features a method of replenishing the water level in an metal-air battery by positioning a hydrogen recombination catalyst in a metal-air battery. The metal-air battery includes (a) an anode, (b) a cathode that includes a metal that reduces oxygen when the battery is in use, and (c) a separator between the anode and the cathode. The hydrogen recombination catalyst cycles between an oxidative stage to oxidize hydrogen to water and a reductive stage to reduce oxygen to regenerate the oxidative properties of the hydrogen recombination catalyst, e.g., when hydrogen encounters the catalyst in the presence of oxygen, the hydrogen is oxidized to water and the catalyst is reduced (the oxidative stage).
The reductive stage occurs when oxygen in the electrochemical cell interacts with a reduced hydrogen recombination catalyst to oxidize the catalyst and thereby restore the oxidative properties of the hydrogen recombination catalyst, i.e., once re-oxidized by oxygen, the catalyst can, in turn, further oxidize hydrogen to water. As a result of the hydrogen recombination catalysts"" ability to periodically cycle through the oxidative and reductive stages, the hydrogen recombination catalyst in a metal air electrochemical cell oxidizes an amount of hydrogen above the theoretical anaerobic hydrogen absorption limit of the catalyst. The theoretical anaerobic hydrogen absorption limit can be determined based on the molar amounts of catalyst material and by assuming that the catalyst undergoes only a single, complete oxidative stage without undergoing a reductive stage. The hydrogen recombination catalyst preferably oxidizes an amount of hydrogen greater than about 2 times the theoretical anaerobic hydrogen absorption limit. At ambient temperature, the hydrogen recombination catalyst preferably oxidizes hydrogen at a rate at least about 0.5 standard cubic centimeter (scc) per gram of hydrogen recombination catalyst per day. Under anaerobic conditions, the hydrogen recombination catalyst preferably can oxidize hydrogen at this rate for at least 130 days. More preferably, the hydrogen recombination catalyst preferably can oxidize hydrogen at this rate for at least 240 days. Under aerobic conditions, the hydrogen recombination catalyst may function indefinitely by combining any hydrogen with oxygen present in the electrochemical cell.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.